The following code makes an unnecessary DeadLetter while executing GracefulStop.

var greeter = system.ActorOf<GreetingActor>("greeter");
greeter.Tell(new Greet("World"));
greeter.GracefulStop(TimeSpan.FromSeconds(1)).Wait();

DeadLetter log is

[akka://MySystem/deadLetters]
Message DeadLetter from akka://S/user/greeter to akka://S/deadLetters was not delivered.
1 dead letters encountered.

This is caused because there is a difference between Akka and Akka.NET.

GracefulStop of Akka
sends Unwatch message only when the target actor is not terminated because it is not necessary.

But GracefulStop of Akka.NET always
sends Unwatch message even when the actor is terminated, which makes DeadLetter message.